Silicones are rapidly becoming ubiquitous components of pharmaceutical and personal care products. Their remarkably high thermal and chemical stability, their unusual ability to resist adhering to other materials and their aesthetically unique and pleasing tactile characteristics have made silicones invaluable raw materials in products designed for topical application to the skin or hair, such as skin treatment preparations, hair conditioners, lipsticks and makeup products. Such products are often desirably formulated in the form of sticks or gels for the elegance and ease of application these product forms offer to the consumer.
The term "silicones" actually encompasses a large and diverse group of polymeric fluids, oils, rubbers, resins and gums generally based on the structural unit --R.sub.2 SiO--, with R being a monovalent organic radical. The type and size of the radical(s) present (e.g., alkyl vs. aryl), the type and degree of polymerization (e.g., linear vs. cyclic), and the presence or absence of crosslinking, all affect the physical and chemical properties of the resultant silicone polymers.
Silicone fluids may be broadly classified as being either volatile or non-volatile, depending on how large a vapor pressure they possess at ambient temperature and pressure. Silicone fluids may also be characterized by their degree of viscosity, where viscosity is defined as resistance to flow and is commonly measured in centistokes. For a particular silicone polymer the viscosity generally increases, and the volatility generally decreases, as the molecular weight of the polymer increases.
Commercially available volatile silicone fluids include hexamethyldisiloxane [(CH.sub.3).sub.3 SiOSi(CH.sub.3).sub.3 ]; methylated cyclic silicones (also referred to as cyclomethicones) which include octamethylcyclotetrasiloxane, an eight-membered ring compound formed from four Si--(CH.sub.3).sub.3 --O groups, and decamethylcyclopentasiloxane, a ten-membered ring compound formed from five Si--(CH.sub.3).sub.2 --O groups; and dimethicones having a viscosity of 10 centistokes or less. Dimethicones are linear silicone polymers conforming to the structure: ##STR1## wherein R is methyl.
Volatile silicone fluids are generally compatible with many of the lipophilic materials commonly employed in topical compositions. They are use in such compositions for their ability to act as co-solvents, as well as to provide a dry, non-greasy feel to the compositions during application to the skin or hair.
Commercially available non-volatile silicone fluids include dimethicones having a viscosity of greater than 10 centistokes, alkylated dimethicones such as cetyl dimethicone (I, R=a mixture of CH.sub.3 and C.sub.16 H.sub.33), and trimethicones such as phenyl trimethicone (II): ##STR2## wherein R is phenyl.
Non-volatile silicone fluids are used in topical compositions to provide a slippery yet non-greasy feel to the compositions, and to provide an occlusive, water-resistant and emollient film to the skin surface upon topical application. They are also used as conditioners in hair treatment products, and as emulsion stabilizers in products formulated as oil-in-water or water-in-oil emulsions. Non-volatile silicone fluids are markedly less compatible than their volatile counterparts with many of the lipophilic materials commonly used in topical compositions.
Silicone gums are non-volatile silicone fluids possessing particularly high viscosities and molecular weights, and include dimethicones having viscosities of about 1,000,000 centistokes. Such gums are used in topical compositions for their unique tactile and film-forming properties, and are available commercially both in gum form and as free-flowing dispersions in silicone fluid carriers.
It is known in the art that polyethylenes are useful gelling agents for volatile silicone fluids. Polyethylenes are polymers of ethylene monomers that conform generally to the formula (C.sub.2 H.sub.4).sub.n, wherein n is usually large enough such that the resultant polymers have average molecular weights of from about 1500 to about 100,000.
U.S. Pat. No. 5,122,519 to Ritter discloses topical gel formulations containing a tetracycline antibiotic, a volatile silicone solvent, an emollient ester co-solvent and a polyethylene gelling agent.
U.S. Pat. No. 5,102,656 to Kasat discloses, inter alia, antiperspirant creams containing a volatile silicone, a polyethylene gelling agent, and an antiperspirant active.
Japanese Published Unexamined Application JP 61069711A discloses, inter alia, solid cosmetic compositions containing a volatile silicone oil, a polyethylene gelling agent and a powder substrate.
The use of polyethylenes as solidifying agents for non-volatile silicone fluids has not been heretofore disclosed. In fact, most commercially available polyethylenes are known to be only minimally soluble in, and hence incompatible with, many of the non-volatile silicones used in topical compositions.
It has now been discovered that low molecular weight polyethylenes, i.e., polyethylenes having an average molecular weight of less than about 1,000, are excellent solidifying agents for silicone fluids. In particular, it has been surprisingly and unexpectedly discovered that such low molecular weight polyethylenes are excellent solidifying agents for non-volatile silicone fluids. It has also been surprisingly and unexpectedly discovered that compositions prepared from low molecular weight polyethylenes and non-volatile silicone fluids are stable, aesthetically elegant and useful in the preparation of novel skin treatment, hair treatment, lipstick and makeup products.
It is therefore an object of the present invention to provide solid silicone compositions suitable for topical application to human skin or hair which contain non-volatile silicone fluids. It is also an object of this invention to provide solid silicone compositions comprising a low molecular weight polyethylene solidifying agent and one or more non-volatile silicone fluids. Further objects of this invention are to provide a method for the preparation of such solid silicone compositions, and to provide skin treatment, hair treatment, lipstick and makeup compositions comprising the solid silicone compositions of the invention.